The objective of this study is to determine the subchronic and chronic toxicity/carcinogenicity study of Isobutene administered by inhalation to Fisher 344 rats and B6C3F1 mice. The contract will include Phase I which is the basic award and includes the development of analytical chemistry, generation and monitoring methods. Phase 2 consists of 1. 13-Week Study: 10 animals/sex/specie shall be treated at each of five dose levels plus controls for 13-weeks 2. 104-Week Study: Fifty animals/sex/specie shall be treated for 104 weeks at 3 dose levels plus controls.